As disclosed in U.S. Pat. No. 4,281,671, to Bynre et al., which is incorporated herein in its entirety by reference, methods are known for the continuous production of tobacco smoke filters wherein a smoke modifying agent is incorporated in the filter product by continuously entraining with the advancing supply of tobacco smoke filtering material a continuous thread or tape carrying the smoke modifying agent. The thread or tape is continuously entrained with the advancing supply of tobacco smoke filtering material before the smoke filtering material is condensed to rod form, whereby the thread or tape becomes incorporated in or on the body of the filter rod and extends continuously longitudinally thereof. The thread or tape employed according to the disclosure in the '671 patent may be of any form of nontoxic material provided that it takes up the smoke modifying agent and releases it subsequently during use of the filter. The thread or tape is preferably a textile material, such as a sewing thread or yarn. The smoke modifying agent incorporated into the thread or tape imparts an additional taste or aroma to the smoke passing through the filter in use. The smoke modifying agent is preferably applied to the thread immediately before it is incorporated in the advancing filtering material.
In the method described in the '671 patent, continuously advancing tow material such as a bonded cellulose acetate filamentary tow, is banded, sprayed with a liquid plasticizer such as triacetin, and then passed through a conventional wrapping garniture in which it is gathered to rod form and enwrapped in a paper wrapper which is secured around the formed rod by an overlapped seam. The wrapped filter rod emerging continuously from the garniture is then severed into individual lengths. At least one thread carrying a controlled amount of smoke modifying agent, applied for example by passage through a solution of the agent, is preferably continuously incorporated into the tow to move in unison therewith after the application to the tow of the liquid plasticizer.
The thread or other continuous element carrying a smoke modifying agent is led into entrainment with the tow via a mandrel, and is preferably positioned near the center and along the entire length of each filter that is cut from the continuous filter rod. The '671 patent recognizes that the thread can be monitored for breaks, for example by using a photoelectric cell device, but no details for how this would be done are provided. Furthermore, although knots are generally formed in the thread when one length of thread is joined to a subsequent length of thread, no mention is made of a process for avoiding the use of a filter rod section having a length of the thread with one or more knots formed therein.